On-road trailers must be equipped with working turn signal, stop and running lights. Early efforts to so equip trailers required accessing the electrical wire harness of the towing vehicle, a laborious and time-consuming task. In order to eliminate the need for connecting the electrical system of a trailer directly to the towing vehicle wire harness, adapter bulbs were developed which could be connected directly to the tail light sockets of the vehicles. The adapter bulbs include wire, or "pigtail," leads for connection to the trailer, which receive power when the adapter bulbs are disposed in the tail light sockets. Such adapter bulbs are disclosed, for example, in U.S. Pat. Nos. 3,137,448; 3,335,389; 3,400,293; and 4,772,209.
Known adapter bulbs have included round bulb bases, since round-base tail light bulbs were long the standard. In recent years, rectangular, or "square" base tail light bulbs have become popular. Typically, rectangular tail light bulbs have two filaments. Sockets for rectangular base tail light bulbs thus have four contact points, two negative and two positive, disposed substantially at each corner of the socket. The four contact points thus form two pairs of substantially diagonally opposed contact points.
The modern rectangular bulb bases present a challenge, however, due to the variability of the arrangement within the socket of the positive and negative connections for the bulb filaments. In one common arrangement, shown in FIG. 1, an "end ground" socket 10 has two positive contact points 12 on one end of socket 10, and two negative contact points 14 at the opposite end of socket 10. In a second arrangement, shown in FIG. 2, a "side ground" socket 16 has positive contact points 12 on one side of socket 16, and the negative contact points 14 on the other side of socket 16.
A known rectangular base bulb 18 for use with either of sockets 10 or 16 typically has first and second filaments 20 and 22, respectively, which are placed in contact (by means of leads, not shown) with the two positive contact points 12 and the two negative contact points 14 in a diagonal or criss-cross manner (see FIG. 3), although the filaments are not necessarily in a criss-cross relationship within the bulb itself. Such rectangular base bulbs 18 will light regardless of the type of socket into which they are inserted. As shown in FIGS. 4a-b, filaments 20 and 22 are each in contact with a positive contact point 12 of the socket into which the bulb is inserted, whether that socket is an "end ground" as shown in FIG. 4a, or a "side ground" as shown in FIG. 4b.
For an adapter bulb to properly function, however, the filament ends that are used for wire lead attachment must contact the positive connections in the rectangular bulb socket. Thus, up to now, adapter bulbs for rectangular bulb sockets have come in two different configurations with two wire lead connection points, each useful with one type of rectangular bulb socket. In addition, adapter bulbs must be oriented correctly within the correct socket type. In FIG. 5, an "end ground" adapter bulb 24 has wire lead connection points 26 on one end as shown. In FIG. 6, a "side ground" adapter bulb 28 has wire lead connection points 26 on one side as shown.
Determining which adapter bulb is suitable for any given rectangular bulb socket, and whether or not the adapter bulb is correctly oriented with respect to the socket, is often difficult. FIGS. 7a-d and 8a-d illustrate the possible configurations of "end ground" and "side ground" rectangular base adapter bulb assemblies, respectively, within rectangular base sockets. The configurations are: (a) Correct bulb, correct socket orientation; (b) correct bulb, incorrect socket orientation; (c) incorrect bulb, orientation 1; and (d) incorrect bulb, orientation 2.
In FIG. 7a, "end ground" adapter bulb 24 is correctly oriented in "end ground" socket 10. Both wire lead connection points 26 are in contact with positive contact points 12, and thus both wire leads (not shown) are working leads. In FIG. 7b, adapter bulb 24 is inserted incorrectly oriented in socket 10. As a result, neither wire lead connection point 26 contacts a positive contact point 12 in socket 10, and so neither wire lead is a working lead. In FIG. 7c, "end ground" adapter bulb 24 is inserted in "side ground" socket 16, with the result that only one wire lead is a working lead. In FIG. 7d, adapter bulb 24 is inserted in socket 16 in reversed orientation. Again, only one wire lead is a working lead. Analogous configurations are shown in FIGS. 8a-d for "side ground" adapter bulb 28.
Thus, only the two configurations in which the adapter bulb is correctly oriented in the correct socket result in a completely functional trailer connection.
A need has existed for a rectangular base adapter bulb that provides wire lead connection points which contact both positive connections within the bulb socket regardless of the arrangement of connections within the bulb socket or the orientation of the adapter bulb with respect to the bulb socket.